Chemical-mechanical planarization (“CMP”) processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 60, a polishing pad 40, and a planarizing liquid 41 on the polishing pad 40. The polishing pad 40 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a fixed abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 41 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the wafer, or the planarizing liquid 41 may be a planarizing solution without abrasive particles that contains only chemicals to etch and/or oxidize the surface of the wafer. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 also has an underpad 25 attached to an upper surface 30 of the platen 20 and the lower surface of the polishing pad 40. In one type of CMP machine, a drive assembly 50 rotates the platen 20 as indicated by arrow A. In another type of CMP machine, the drive assembly reciprocates the platen back and forth as indicated by arrow B. Since the polishing pad 40 is attached to the underpad 25, the polishing pad 40 moves with the platen 20.
The wafer carrier 60 has a lower surface 63 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 64 positioned between the wafer 12 and the lower surface 63. The wafer carrier 60 may be a weighted, free-floating wafer carrier, or an actuator assembly 61 may be attached to the wafer carrier to impart axial and/or rotational motion (indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 60 presses the wafer 12 face-downward against the polishing pad 40. While the face of the wafer 12 presses against the polishing pad 40, at least one of the platen 20 or the wafer carrier 60 moves relative to the other to move the wafer 12 across the planarizing surface 42. As the face of the wafer 12 moves across the planarizing surface 42, the polishing pad 40 and the planarizing liquid 41 continually remove material from the face of the wafer 12.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 μm. Focusing photo-patterns of such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Thus, CMP processes must create a highly uniform, planar surface.
One problem with conventional CMP processing techniques is that the planarized surface of the wafer may not be sufficiently uniform due to nonuniformities that may develop in the planarizing surface of the polishing pad during planarization. One conventional approach to addressing this problem is to firmly attach the polishing pad to the platen to decrease the likelihood that the polishing pad will warp or wrinkle as the wafer carrier and substrate move across the planarizing surface. For example, in one conventional approach, the polishing pad may be attached to the platen with a high-strength adhesive. One drawback with this approach is that the planarizing surface of the polishing pad typically wears out during normal use and the polishing pad must therefore be replaced. It may be difficult and time consuming to remove the polishing pad and the high-strength adhesive from the platen, rendering the CMP machine inoperable for extended periods of time.
One conventional approach to addressing the foregoing problem is to manufacture a sheet of polishing pad material and stretch it across the platen from one side to the other. As the polishing pad wears, it is incrementally moved across the platen in the manner of a conveyor belt to present an unworn planarizing surface to the wafer. Such a device is manufactured by Obsidian, Inc. of Fremont, Calif. One problem with this approach is that the tension in the sheet may not be sufficient to keep it flat against the platen. Accordingly, the sheet may tend to wrinkle or fold upon itself under the pressure exerted by the wafer carrier and the wafer.